1. Technical Field
The present disclosure relates to an apparatus for measuring temperature distribution along an optical fiber by using Raman scattered light.
2. Related Art
A DTS (Distributed Temperature Sensor) has been used for measuring temperature distribution along an optical fiber. The DTS is an apparatus for measuring temperature distribution along an optical fiber by using backscattered light generated in the optical fiber. The backscattered light includes Rayleigh scattered light, Brillouin scattered light and Raman scattered light, for example. The Raman scattered light, which has strong temperature dependence, is used for temperature measurement. In the case of the temperature measurement by using the Raman scattered light, anti-Stokes light (AS light) having a shorter wavelength than incident light and Stokes light (ST light) having a longer wavelength than the incident light are used.
The DTS measures the intensity Ias of the AS light and the intensity Ist of the ST light to calculate the temperature based on the intensity ratio. Thus, the temperature distribution along the optical fiber is measured. The DTS is used in fields such as temperature management of a plant installation, disaster prevention, temperature management of an electric cable, and air conditioning of a server room or a building.
The principle of the DTS is shown in FIG. 4. In measurement of the temperature distribution by the DTS, a temperature distribution measurement unit 100 outputs a light pulse to an optical fiber 101 (optical fiber for sensor). As shown in FIG. 5, the temperature distribution measurement unit 100 includes a pulse generation unit 102, a light source 103, a directional coupler 104, an optical filter 105, a first optical-electrical converter 106, a second optical-electrical converter 107, a first amplifier 108, a second amplifier 109, a first AD converter 110, a second AD converter 111, an averaging circuit 112, a calculation unit 113, a temperature correction unit 114, and a reference temperature unit 115.
The pulse generation unit 102 causes the light source 103 to generate pulse light with timing in synchronization with the averaging circuit 112. The generated pulse light is input to the optical fiber 101 through the directional coupler 104. As shown in FIG. 4, Raman scattered light is generated almost everywhere in the optical fiber 101. The generated Raman scattered light is input to the temperature distribution measurement unit 100. The Raman scattered light input to the temperature distribution measurement unit 100 is guided by the directional coupler 104 to the optical filter 105.
The Raman scattered light is split by the optical filter 105 into anti-Stokes light (AS light) and Stokes light (ST light). The ST light is opto-electrically converted by the first optical-electrical converter 106, amplified by the first amplifier 108, and converted by the first AD converter 110 into a digital signal. The AS light is opto-electrically converted by the second optical-electrical converter 107, amplified by the second amplifier 109, and converted by the second AD converter 111 into a digital signal.
The averaging circuit 112 performs averaging processing for noise reduction. The calculation unit 113 calculates the ratio of the AS light intensity Ias to the ST light intensity Ist (Raman intensity ratio). The Raman intensity ratio is proportional to the temperature of the optical fiber 101. Therefore, the temperature distribution along the optical fiber 101 can be measured based on the Raman intensity ratio.
As shown in FIG. 5, the reference temperature unit 115 is provided between the directional coupler 104 and the optical fiber 101. An optical fiber is coiled in the reference temperature unit 115. The reference temperature unit 115 is provided with a highly-accurate thermometer 115s typified by a platinum resistance temperature sensor. The thermometer 115s measures the reference temperature and outputs a measurement result to the calculation unit 113. The calculation unit 113 calculates the temperature distribution along the optical fiber based on the reference temperature and the Raman intensity ratio.
A signal on a time domain of the Raman intensity ratio can be obtained by inputting the light pulse to the optical fiber 101 as shown in FIG. 4. The time corresponds to a location along the optical fiber. The calculation unit 113 obtains the Raman intensity ratio of the whole length along the optical fiber 101. The calculation unit 113 performs predetermined calculations by using the Raman intensity ratio and the reference temperature. As a result, the temperature distribution of the whole length of the optical fiber 101 can be obtained. The temperature correction unit 114 performs predetermined correction processing with respect to the temperature distribution along the optical fiber 101 obtained by the calculation unit 113.
As shown in FIG. 4, in a spectrum of the Raman scattered light, the anti-Stokes light (AS light) appears on the shorter-wavelength side (wavelength: λ0-λX) of the Rayleigh light (wavelength: λ0). In addition, he Stokes light (ST light) appears on the longer-wavelength side (wavelength: λ0+Δλ′) of the Rayleigh light.
In this manner, the temperature distribution measurement unit 100 uses the Raman scattered light to obtain the temperature with respect to each location along the optical fiber 101 corresponding to the time domain. As a result, the temperature distribution of the whole length of the optical fiber 101 is measured. If there is a hot section HT as shown in FIG. 4, the temperature of the optical fiber 101 is high in the vicinity of the hot section HT. Therefore, temperature distribution where the temperature of a part of the optical fiber 101 is increased is obtained as shown in FIG. 4.
In the case of the configuration shown in FIGS. 4 and 5, only one end of the optical fiber 101 is connected to the temperature distribution measurement unit 100. That is, the above-mentioned measurement is single-ended temperature distribution measurement with respect to the optical fiber 101. Here, loss of the AS light intensity Ias and the ST light intensity Ist is caused within the optical fiber 101 while the Raman scattered light propagates from a location of generation to the near end of the optical fiber. Therefore, in the single-ended measurement, the Raman intensity ratio is corrected by using the loss ratio of Ias to Ist (Raman loss ratio (loss profile)) of the optical fiber.
For this reason, the loss profile is recognized in advance in the single-ended measurement. However, if the optical fiber 101 includes a plurality of different kinds of optical fibers having different Raman loss ratios each of which is connected to each other, the Raman loss ratio is different depending on a location along the optical fiber 101. In this case, an accurate loss profile is hard to recognize.
FIG. 6 is a diagram illustrating an apparatus for performing double-ended measurement along the optical fiber temperature. In the double-ended measurement, either one of the near end and the far end of the optical fiber 101 is selected, and the selected one is connected to an optical switch 120 that is connected to the temperature distribution measurement unit 100.
The optical switch 120 has a channel A (CH-A) and a channel B (CH-B). The channel A is connected to one end of the optical fiber 101, and the channel B is connected to the other end of the optical fiber 101. In the double-ended measurement, both of measurement by using the channel A and measurement by using the channel B are performed in order to obtain Ias and Ist. Thereafter, measurement results are synthesized (obtain geometric mean). As shown in FIG. 7, a distance from the channel A to a location X in the optical fiber 101 is defined as X. The Raman intensity ratio that is measured with respect to the location X is defined as G(X). In this case, in the double-ended measurement, G(X) can be expressed by the following Equation 1.
                                              ⁢                  〈                      Equation            ⁢                                                  ⁢            1                    〉                                                                              G          ⁡                      (            X            )                          =                                                            Ias                ⁢                                                                  ⁢                                  A                  ⁡                                      (                    X                    )                                                  ×                                  L                  ⁡                                      (                    X                    )                                                                              Ist                ⁢                                                                  ⁢                                  A                  ⁡                                      (                    X                    )                                                                                ×                                                    las                ⁢                                                                  ⁢                                  B                  ⁡                                      (                                          m                      -                      X                                        )                                                  ×                                  L                  ⁡                                      (                                          m                      -                      X                                        )                                                                              Ist                ⁢                                                                  ⁢                                  B                  ⁡                                      (                                          m                      -                      X                                        )                                                                                                          (                  Equation          ⁢                                          ⁢          1                )            
Here, IasA(X) is Ias at the location X responding to the light pulse from the channel A;
IstA(X) is Ist at the location X responding to the light pulse from the channel A;
IasB(m−X) is Ias at the location X responding to the light pulse from the channel B;
IstB(m−X) is Ist at the location X responding to the light pulse from the channel B;
m is the total length of the optical fiber 101;
L(X) is the Raman loss ratio from the channel A to the location X; and
L(m−X) is the Raman loss ratio from the channel B to the location X.
Note that IasA(X)/IstA(X) and IasB(m−X)/IstB(m−X) in Equation 1 are equal to each other because they both are the pre-loss Raman intensity ratio in the optical fiber with regard to the same location. The above-mentioned same value is defined as G0(X). In this case, the following Equation 2 can be obtained.<Equation 2>G(X)=G0(X)×√{square root over (L(X)×L(m−X))}{square root over (L(X)×L(m−X))}  (Equation 2)
As mentioned above, L(X) is the Raman loss ratio from the channel A (one end of the optical fiber 101) to the location X, and L(m−X) is the Raman loss ratio from the location X to the channel B (other end of the optical fiber 101). The result of multiplication of L(X) and L(m−X) corresponds to the Raman loss ratio from the one end to the other end of the optical fiber 101 (i.e. the Raman loss ratio with regard to the total length of the optical fiber 101). Accordingly, the term “L(X)×L(m−X)” in Equation 2 takes a constant value independent of the location X. When “L(X)×L(m−X)” is denoted by “Ltotal”, the above-mentioned Equation 2 can be written as the following Equation 3.<Equation 3>G(X)=G0(X)×√{square root over (Ltotal)}  (Equation 3)
As mentioned above, the parameter Ltotal is the Raman loss ratio with regard to the total length of the optical fiber 101 and independent of the location X in the optical fiber 101. Therefore, there is no need to recognize the above-mentioned loss profile in the double-ended measurement. The temperature distribution along the optical fiber is accurately measured as long as the parameter Ltotal being a fixed value is recognized.
Such a technique that measures the temperature distribution along the optical fiber based on the intensity ratio of the Stokes light intensity Ist to the anti-Stokes light intensity Ias is disclosed in, e.g., JP-A-2008-249515.